1. Field of the Invention
The present invention relates to a pressure switch which is used for ON-OFF control of an electric circuit in response to a predetermined pressure.
2. Description of the Related Art
Pressure switches of various types are widely used in, for example, refrigeration systems. Well-known switch types include high- and low-pressure-cut off types, high- and low-pressure-on types, double-action type, and triple-action type. The high-pressure-cut off type serves to cut off a certain device in the refrigeration system from the power supply when the pressure of a refrigerant in the system exceeds a predetermined level. The low-pressure-cut off type serves to cut off a certain device in the refrigeration system from the power supply when the pressure of the refrigerant is reduced below a predetermined level. The high- and low-pressure-on types acts reversely in comparison with above described two cut off types. The double-action type serves to turn on or off a certain device within a predetermined range of refrigerant pressure. The triple-action type serves to turn on or off two devices within two different predetermined ranges of refrigerant pressure.
The pressure switches of the high-pressure-cut off type are used, for example, to cut off a compressor from the power supply when the pressure of the refrigerant in a refrigeration system exceeds the predetermined level. The pressure switches of the high-pressure-on type are used, for example, in a refrigeration system having a condenser with a cooling fan to turn on the cooling fan of the condenser of the refrigeration system when the pressure and temperature of the refrigerant in the system exceed predetermined levels.
FIG. 1 schematically shows an arrangement of a typical conventional pressure switch of the low-pressure-cut off type. This type of switches are on the market by the assignee of this application. In FIG. 1, the inner space of housing 10 is divided into two compartments by means of diaphragm 12, which is formed of a flexible polyimide film. One of the compartments serves as pressurized fluid operating chamber 16 which communicates with pressurized fluid inlet hole 14, while the other compartment serves as switch mechanism chamber 18. In chamber 18, movable terminal supporting member 20 is pressed against diaphragm 12 by urging means 22, so that supporting member 20 is located in an OFF position where it is away from fixed terminals 24 in chamber 18. As the pressure of a pressurized fluid introduced into pressurized fluid operating chamber 16 overcomes the urging force of urging means 22, supporting member 20 is moved to an ON position where movable terminal 26 is pressed on fixed terminals 24.
FIG. 2 schematically shows an arrangement of a typical conventional pressure switch of the double-action type constructed by combining the construction for the low-pressure-cut off type described above and the construction for the high-pressure-cut off type. This type of switches are widely known by U.S.P. 4,593,166. In FIG. 2, snap disk supporting member 30 in switch mechanism chamber 18 is pressed against diaphragm 12 by urging means 22. Snap disk 32 is disposed in a tray-shaped depression on the diaphragm-side end face of supporting member 30, and projects at its center portion toward diaphragm 12. Movable terminal driving projection 34, which is fixed on the central portion of disk 32, penetrates supporting member 30. The projecting end of driving projection 34 faces movable terminal 36, which extends along the urging-means-side end surface of supporting member 30 in switch mechanism chamber 18 and is located at its OFF position where it is away from fixed terminals 24. When the pressure of the pressurized fluid introduced into pressurized fluid operating chamber 16 exceeds a first predetermined level, supporting member 30 is moved downward against the urging force of urging means 22 to press movable terminal 36 to its 0N position where its is pressed on fixed terminal 24. When the pressure further rises and exceeds a second predetermined level, snap disk 32 is transformed to project its center portion toward the bottom surface of the tray-shaped depression on the diaphragm-side end surface of snap disk supporting member 30. Thus, the projecting end of movable terminal driving projection 34 presses movable terminal 36, thereby moving it to an OFF position where it is away from fixed terminal 24.
Polyimide-film diaphragm 12, which is used in the two prior art examples described above, has best mechanical properties for use in pressure switches. The polyimide film, however, is preamble to some of various refrigerants used in various refrigeration systems, so that the pressure switches having the polyimide-film diaphragm can be used only in a refrigeration system employing a refrigerant permeability of which to diaphragm 12 is too little to arise a problem on practical use, e.g., a car air conditioner which employs the R12 as a refrigerant.
Recently has been raised, however, the problem of destruction of the ozone layer in the atmosphere by fron gas (especially, chlorofluorocarbon). Accordingly, a restriction of the manufacture and sale of refrigerants R11, R12, R113, R114, and R115, which are composed mainly of chlorofluorocarbon, have started on a world-wide scale. It has been proposed that the R22, R134 or a fluid produced by mixing R500 and any other components should be used as the refrigerant for the future. Thus, when those refrigerants which are composed mainly of chlorofluorocarbon are used, it is strictly required to hermetically seal it.
If the aforementioned pressure switch, employing polyimide-film diaphragm 12 which has high permeability to all the refrigerants except the R12, are used in these circumstances, the refrigerant may leak from the refrigeration system unless switch mechanism chamber 18 of housing 10 is hermetically sealed. If chamber 18 is hermetically sealed, however, the switch mechanism in chamber 18, which is adapted to be operated by the difference in pressure between chamber 18 and pressurized fluid operating chamber 16 divided by diaphragm 12, is prevented from operating normally.
FIG. 3 schematically shows an arrangement of a pressure switch conventionally used in a refrigeration system which employs the R22, R502, or R500 as a refrigerant. This type of switches are widely known by U.S.P. 3,584,164, and are on the market by the assignee of this application.
In this pressure switch, diaphragm 40 is formed of a metal sheet, and its peripheral edge is welded to the inner surface of the inner space of housing 10. Such diaphragm 40 elastically deforms to project at its center portion toward switch mechanism chamber 18 when the pressure of a fluid introduced into pressure fluid operating chamber 16 in housing 10 exceeds a predetermined level. Thereupon, movable terminal driving projection 42 fixed to the lower surface of diaphragm 40 moves movable terminal 44 from its ON position, where it presses on fixed terminal 24, to its OFF position, where it is away from the fixed terminal 24. When the fluid pressure inside chamber 16 is lowered to a pressure level not higher than the predetermined level, diaphragm 40 is restored, by its own elastic force, to its initial configuration, so that it projects at its center portion toward pressure fluid operating chamber 16. The pressure switch of this construction has an advantage in its quick switching operation, and can be effectively used in a refrigeration system which employs the refrigerant under a relatively high pressure.
In the pressure switch constructed in this manner, however, diaphragm 40 is distorted by heat which is produced while the peripheral edge of the diaphragm is being welded. In a final process of the manufacture of the switch, therefore, changing in the operating performance of diaphragm 40, due to the thermal distortion, must be corrected. Moreover, the welding work is complicated, and therefore, entails an increase in the manufacturing costs of the pressure switch. And, the pressure switch of this construction has a difficulty to construct the double-action type pressure switch as shown in FIG. 2 and the triple-action type pressure switch.